Unplug-And-Play Hydraulic Workholding

An OEM and contract shop have found value in an unconventional, untethered hydraulic workholding method that uses mechanical energy transfer to charge a fixture's sealed hydraulic circuit. Free and easy HMC pallet movement is the result.

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Because no two fixtures are exactly the same, even those designed to hold the same part, probing is used to identify a previously machined feature to accurately position the part before machining successive operations.

3D CAD is helpful is visualizing the tubing, valves and other components that must be installed on a hydraulic workholding fixture. The image on the bottom is a sectioned view of the top image, showing the location of hydraulic circuit components inside the tombstone.

There are three key parts of the mechanical energy transfer system—fixture-mounted booster, activator wand and activator pump. After parts are attached to the fixture, the operator will insert the wand (a remote cylinder) into the booster installed on the fixture. The pump will extend the wand which pushes a piston inside the booster and pressurizes the circuit.

Manufacturers that use hydraulic workholding to clamp parts for machining fit a standard profile. Their job runs typically are medium to high in quantity and their machining processes must offer the highest throughput and lowest scrap rate. That means speedy, repeatable part clamping is essential.

An OEM in Iowa differs from that profile, if only slightly, in that it uses a distinctive method to charge the hydraulic circuit on pallet-mounted fixtures used for its horizontal machining centers (HMCs). A nearby contract shop that supplies these fixtures to the OEM also uses the atypical system for some of its high-volume machining jobs.

Rather than using compressed air to charge a fixture's hydraulic circuit to secure parts for machining, this alternate technique is based on a mechanical transfer of energy. It uses a remote cylinder device that is temporarily attached to the fixture and pressurizes a completely sealed hydraulic circuit (think in terms of a car's braking system). Once parts are secured, the remote device is removed, while parts remain firmly clamped and pallet movement unrestrained.

Riding The Rail

The companies mentioned are located within a short drive of each other in central Iowa—Sauer-Danfoss in Ames and Mid-America Manufacturing (MAM) in the city of Nevada. Sauer-Danfoss produces hydraulic components for mobile equipment, so it's no surprise that the company uses fluid power to hold the pumps, motors and valves it produces at this facility during machining. MAM is both a contract machine shop and a custom fixture manufacturer. It supplies a number of fixtures to Sauer-Danfoss with the mechanical energy transfer system, and also uses that system on some of the seven HMCs in its facility.

Sauer-Danfoss has four separate Makino pallet systems that serve a number of HMCs. Because pallets are shuttled back and forth between loading station and machine, and because the pallets are rotated within the machine, it is impractical to have each fixture leashed with continuously attached hydraulic lines for part clamping. So the company began using a quick-disconnect hydraulic system to charge the circuit on each fixture, but found that dirt and debris were prone to enter into the hydraulic stream, leading to maintenance downtime and seal replacements.

Sauer-Danfoss has since standardized on the mechanical energy transfer method to power the hydraulic workholding circuit on the fixture. Tom Taylor, tool designer, says that his Ames facility has nearly 70 fixtures with the mechanical energy system that are used on 36 HMCs. Let's take a closer look at this hydraulic workholding technique and its main components.

Mechanical Push

The mechanical energy transfer system was developed by Enerpac (Milwaukee, Wisconsin). The system has three key components—hydraulic booster, activator wand and activator pump (see the schematic to the right). The wand is a high-pressure remote hydraulic cylinder that is plumbed to a footswitch-activated pump. After the fixture is cleaned of debris and the operator mounts a new part, a shut-off valve—located in the circuit just after the fixture-mounted booster—is opened. The operator then inserts the wand into the booster. When the 10,000-psi pump is activated, the wand's cylinder extends into the booster, pushing a piston which pressurizes the fixture's hydraulic circuit (circuit pressure is typically half that of the wand). When peak circuit pressure is reached, the operator will close the shut-off valve which prevents fluid from flowing back into the booster and keeps the parts firmly clamped even after the wand is depressurized and removed from the booster. The pallet is now ready to be shuttled to an HMC.

To remove parts from the fixture after machining, the operator will open the shut-off valve to allow fluid back into the booster. The hydraulic clamps will then move up and rotate out of the way (in the case of swing clamps), allowing the part to be removed.

Considering the number of pallets and machines in each pallet system, part positioning variance is bound to be an issue. To account for any differences, Sauer-Danfoss probes parts once the pallet is shuttled into the machine. The probe will touch off on a previously machined known location (like a bolt hole) in order to obtain the part's actual position inside the machine.

Hydraulic Consistency

MAM uses hydraulic workholding for its contract work if the volume is sufficient, according to Jason Elsberry, design manager. "We use hydraulic workholding primarily for consistency," says Mr. Elsberry. "You can supply clamping torque values and tell operators to use torque wrenches when clamping, but that doesn't always happen. Every operator will have a different sequence to clamp the part, which can also make a difference as to whether a part is secured properly."

For Sauer-Danfoss and other high-production manufacturers that use hydraulic workholding, fast clamping speed and reduced setup time are equally important, especially in lean manufacturing production schemes. The mechanical energy transfer method of hydraulic workholding offers Sauer-Danfoss a good base from which other setup-reducing techniques can sprout.

Getting A Visual

MAM has a lot of experience designing fixtures using the mechanical energy transfer system. As a good part design is driven by innovation and creativity, so too is a good fixture design. Part designers are constrained by physical limitations of the final assembly, while fixture designers are constrained by part geometry, machine tool envelope, the total number of parts to be held and the number of machining operations the parts will receive. Hydraulic workholding fixtures, especially those using the mechanical energy transfer system, add another layer of complexity because of the piping, valves and other necessary circuit components.

One of the ways that MAM speeds the design of its congested fixture projects is through a 3D CAD system from Solidworks (Concord, Massachusetts). One of the biggest advantages of 3D CAD in terms of fixture design is the capability to visualize the layout of the hydraulic circuit components. A 3D CAD fixture model can be sectioned in any desired plane to quickly check for fixture component interferences. (Many workholding component suppliers assist in speeding the design process by offering 3D CAD drawings on their Web sites that can be download and imported into the fixture drawing, eliminating some drawing time.)

MAM tries to conserve fixture space by using manifolds where possible. External rigid tubing is less expensive than manifolds, but it has drawbacks. One is the tendency to collect chips. The other is the possibility that the machine's cutting tool could nick the tubing, causing a leak. Manifolds offer a cleaner fixture design, and manifold-mounted components are typically easier to replace. Similar to the manual-versus-power workholding debate, the choice to use manifolds depends on the size of the production run and time allotted for fixture design and manufacture. "We will try to use a plate or portion of the tombstone weldment itself as a manifold," Mr. Elsberry explains.

Fixture weight is another design constraint, as the total weight of finished fixture loaded with hydraulics and parts to be machined must not exceed the capacity of the machine's pallet. The 3D system can not only automatically calculate total fixture weight, but also locate the fixture's center of gravity. This is helpful in determining where to locate hoist points in order to maintain fixture balance during installation onto a pallet.

Clamping Choice

The decision of how to position, support and secure a part for machining generally boils down to cost—not just in terms of the fixture itself, but the overall part production cost. This is driven by factors such as production quantity; number of parts per fixture; material, machine and fixture costs; and load/unload and run times.

While these variables should all be considered when determining the best workholding method for an application, there are some general guidelines. Mechanical clamping is more time-consuming than hydraulics because the operator has to manually clamp each part. But because of the higher cost of a hydraulic system, mechanical clamping is often used for low-volume production runs. The cost for quick-clamping hydraulic systems can often be justified for high-volume jobs. As labor costs continue to rise, hydraulic workholding tends to move more toward the lower end of the part volume scale.

Clamping accuracy is also a factor. Mechanical clamping lacks the repeatability of hydraulic clamping. Variance in torque can result in significant differences in part height and position on a fixture, but this variance may be acceptable if surface finishes and tolerances are not critical or the parts being machined aren't likely to distort during clamping. One way to increase repeatability of manual clamping is to specify torque values for each clamp and insist that operators use a torque wrench.

Hydraulic workholding products offer repeatable clamping force. This type of accuracy may be necessary when machining a surface requiring tight tolerances where a slight variation in clamping force could result in part movement or deflection that could exceed the required overall tolerance.

Hydraulic workholding should also be considered if a goal is to maximize the number of parts that can be installed on a fixture. Compact, standard hydraulic components optimize the use of clamping space and allow the possibility to clamp in manually inaccessible areas.

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